Astronomers have spent much time analysing how stars form and how they develop. One problem was to explain what happened to a massive star at the end of its life. In 1967, the term "black hole" was used to describe one type of object that is left when a massive star dies.
Four years later, Cygnus X-1 was found, the first candidate for a black hole.
Although we cannot see black holes, we can detect or guess the presence of one by measuring its effects on objects around it. The following effects may be used:
- Mass estimates from objects orbiting a black hole or spiraling into the core
- Gravitational lens effects
- Emitted radiation
Many black holes have objects around them, and by looking at the behavior of the objects you can detect the presence of a black hole. You then use measurements of the movement of objects around a suspected black hole to calculate the black hole's mass.
What you look for is a star or a disk of gas that is behaving as though there were a large mass nearby. For example, if a visible star or disk of gas has a "wobbling" motion or spinning AND there is not a visible reason for this motion AND the invisible reason has an effect that appears to be caused by an object with a mass greater than three solar masses (too big to be a neutron star), then it is possible that a black hole is causing the motion. You then estimate the mass of the black hole by looking at the effect it has on the visible object.
For example, in the core of galaxy NGC 4261, there is a brown, spiral-shaped disk that is rotating. The disk is about the size of our solar system, but weighs 1.2 billion times as much as the sun. Such a huge mass for a disk might indicate that a black hole is present within the disk.
Einstein's General Theory of Relativity predicted that gravity could bend space. This was later confirmed during a solar eclipse when a star's position was measured before, during and after the eclipse. The star's position shifted because the light from the star was bent by the sun's gravity. Therefore, an object with immense gravity (like a galaxy or black hole) between the Earth and a distant object could bend the light from the distant object into a focus, much like a lens can.
When material falls into a black hole from a companion star, it gets heated to millions of degrees Kelvin and accelerated. The superheated materials emit X-rays, which can be detected by X-ray telescopes such as the orbitingChandra X-ray Observatory.
Black holes have incredibly strong gravity which pulls in anything that comes close enough. Anything pulled in beyond the event horizon will be squashed to near infinite density and never escapes.
Inside a Black hole
Space and time are highly distorted inside a black hole. Anyone who falls into a black hole, by chance, would be stretched to resemble spaghetti, as gravity pulled more on the feet than the head. This is called as being "Spaghetified". An observer watching the person fall would also see time running slower as the person fell towrds the event horizon.
It is important to remember that black holes are not cosmic vacuum cleaners -- they will not consume everything. So although we cannot see black holes, there is indirect evidence that they exist. They have been associated withtime travel and worm holes and remain fascinating objects in the universe.